Locally Controlled Air Movement Preferred in Warm Isothermal Environments

Air movement is one of the six main variables determirzing human thermal comfort; air temperature, relative humidity, mean radiant temperature, metabolic rate, and clothing insulation are the others. Recently, HVAC design in;zovations, energy conservation concerns, and new laboratory data on fan cooling and drafts have brought substantial ~tttention to the issue of acceptable levels of air movement in qO’ice environments. Thermal comjort standards for indoor occupancy include air movement limits that are constructed from often conflicting evidence and are frequently difficult to apply. A primary reason is that, while air movement can provide desirable cooling in "warm" conditions, it can also increase the risk of unacceptably cool drafts. The transition zone from desirable cooling to uncomfortable draft is a complicated function of physics, physiology, and human expectation. This work focuses on air movement for cooling in the expected temperature range, 25.5°C to 28..5°C, of this transition zone. Fifty-four human subjects were given control of the air supply velocity from a desk fan (FAN), a floor-mounted diffuser (FMD). and a desk-mounted diffuser (DMD) single ambient air temperature. The subjects were asked to adjust the air movement as they pleased to make themselves comfortable. These tests encompassed the full temperature range of the "transition zone," 25.5°C to 28.5°C. Physical measurements of the environment were made and subjective votes collected, including thermal sensation, thermal preference, wor.’~ area preferences, personal control preferences, and health characteristics. A model that predicts the percentage of satisfied people (the PS model) as a function air temperature and air movement in warm conditions is proposed. INTRODUCTION Thermoregulation and Sensation Air movement is one of six main variables affecting human thermal comfort. The other five include three physical variables--air temperature, mean radiant temperature, and relative humidity--and two physiological variables-metabolic rate and clothing insulation. The concept of air movement encompasses not only air velocity but also the fluctuations in air velocity over time. The human thermoregulatory system is responsible for regulating the heat balance of the body, maintaining a core setpoint of 37°C within the constraints of the six variables given above. The body’s thermoregulation is very similar to a building’s HVAC system, which controls different zones of a building with interior setpoints and the constraints of outside weather, internal loads, and shell insulation. Skin and core temperatures for the body are regulated to release metabolic heat through respiration and also via convective, radiative, and evaporative heat transfer through the skin. The convective heat transfer component varies with skin surface temperature, air temperature, and local air motion° Thus, the body’s skin surface temperature is partly determined by the rate of convective heat transfer and, hence, local air movement. Extensive laboratory studies have shown that the thermal sensation vote, an important method for measuring thermal comfort, is closely related to skin temperature in cool and comfortable conditions. Moisture on the skin has an increased effect on thermal sensation in warm and hot conditions, particularly after sweating mechanisms have been triggered. Local air movement is an important factor in thermal comfort and has been incorporated in comfort standards from their inception. Marc Fountain is a researcher, Edward Arens is professor of architecture, Fred Bauman is a research specialist, and Katsuhiro Miura is a visiting researcher, Center for Environmental Design Research, University of California, Berkeley. Richard e Dear is a research associate, Climatic Impacts Centre, Macquarie University, Sydney, Australia. ASHRAE Transactions: Symposia 937